The exhaust gas discharged from internal-combustion engines in a vehicle such as bus, truck or the like, or a construction machine and the like contains particulates exerting upon an environment or human body, so that it is demanded to develop a technique removing the particulates. For example, as one of such techniques, there is a honeycomb structural body (filter) for the purification of the exhaust gas as shown in FIG. 1 wherein the exhaust gas is passed through porous ceramic members to catch and remove the particulates.
As one example of the honeycomb structural body for the purification of the exhaust gas, there is a ceramic honeycomb filter 20 as shown in FIG. 2 wherein a plurality of square-pillar shaped porous ceramic members (units) 30 are combined through seal material layers 23 to constitute a ceramic block 25 and a seal material layer 24 for preventing leakage of an exhaust gas is formed around the ceramic block 25. The porous ceramic member 30 has a function as a filter wherein the particulates are caught by partition walls when the exhaust gas passes through partition walls 33 separating the plurality of the through-holes 31 arranged side by side in the longitudinal direction (wall flow).
The through-holes 31 formed in the porous ceramic member (unit) 30 are clogged at either end portions of inlet side and outlet side for the exhaust gas with a sealing material 32 (preferably in the form of checkered patter) as shown in FIG. 2b, in which the exhaust gas flown from one end portion of the through-hole 31 passes through the partition wall 33 separating the through-hole 31 and flows out from the other end portion of adjoining through-hole 31.
Moreover, the seal material layer 24 disposed on the outer periphery for preventing the leakage of the exhaust gas from the outer peripheral portion of the ceramic block 25 when the honeycomb filter 20 is disposed in an exhaust pipe of an internal-combustion engine as previously mentioned.
Such a ceramic honeycomb filter 20 is now used in large-size vehicles, vehicles provided with a diesel engine and the like because it is excellent in the heat resistance and easy in the regeneration treatment.
In the honeycomb filter 20 for the purification of the exhaust gas, it has hitherto been mainly made to adjust the catching efficiency and the pressure loss by adjusting pore size and pore distribution of pores produced in the porous sintered body (partition walls) or adjusting wall thickness and pore distribution thereof.
For example, Japanese Patent No. 3272746 (hereinafter referred to as article 1) discloses a diesel particulate filter having an average pore size of 1-15 μm and a standard deviation of pore size distribution represented by a common logarithm of the pore size of not more than 0.20.
Also, Internal Publication WO 02/26351 (hereinafter referred to as article 2) discloses a catalyst-carried filter having an average pore size of 10-250 μm and a standard deviation of pore size distribution represented by a common logarithm of the pore size of not more than 0.40.
Furthermore, JP-A-2001-269522 (hereinafter referred to as article 3) discloses a ceramic sintered body filter wherein the wall thickness is made thick in the production of the filter using material having a plurality of larger pores and the wall thickness is made thin in the production of the filter using a material having a plurality of small pores.
Moreover, JP-A-2003-1029 (hereinafter referred to as article 4) discloses a porous ceramic honeycomb filter having a porosity of cell wall of 55-75%, an average pore size of 10-40 μm and a surface roughness (maximum height Ry) of not less than 10 μm.
However, the exhaust gas filters described in the articles 1 and 2 tend to make the surface roughness small because the pore size distribution of the filter becomes extremely uniform. In this case, as the surface roughness becomes small, the surface roughness of the catalyst coating layer becomes also small, so that there is a problem that the reaction site of the catalyst coating layer with the particulates becomes less and the supply of oxygen is insufficient and hence the catalytic reaction becomes insufficient
In the ceramic filter disclosed in the article 3, the distribution amount of the pores and the wall thickness are defined, and it is described to arrange fine pores to the thin wall and large pores to the thick wall, but pores having approximately the same size are dispersed at a uniform density, so that the surface roughness becomes substantially small likewise the articles 1, 2.
Further, the honeycomb filter disclosed in the article 4 is insufficient in the countermeasure to the pressure loss or the like because the pore distribution is not examined.